Would it be possible to have the F# 4.0 bundle also install in VS2013 in addition to VS2015?

I'm afraid that previewing F# r.0 on VS 2013, is not planned. There is a lot of work involved in order to test and release each target platform. Given that Vs 2015 preview is freely available, it is more efficient for us to preview on that.

The VS F# Tools that are VS Addins are built and tested with VS 2015 toolsets, and reference VS 2015 assemblies, so they won't just work with VS 2015. Since it is open source and the open source code includes a VSIX build mechanism, you could always build it
and test it for yourself. I'm not sure that we would take a PR to enable it, because It's not a scenario we want to build testing machinery for. We would rather spend our resources adding value to our mainline scenarios.